1. Field of the Invention
Example embodiments of the present invention are directed to non-volatile memory devices and a method thereof, and more particularly to non-volatile memory devices and a method of obtaining a higher storage capacity per cell area in the non-volatile memory devices.
2. Description of the Related Art
A conventional portable electronic appliance may have a higher capacitance even as a size of the portable electronic appliance is reduced. Due to the reduced size and higher capacitance of portable electronic appliances, non-volatile memory devices may be used within portable electronic appliances to attain higher integration and higher capacitance. However, the higher integration may be achieved by forming higher integration patterns, which may not scale as portable electronic appliances are reduced further in size due to limits of photolithographic processes.
For example, a NOR-type flash memory device may include a cell area of at least 2F×3F, or 6F2, to process 2-bit data. Here, 1F may correspond to a width of a word line. Therefore, the cell area required for processing unit-bit data may be relatively high.
A conventional non-volatile memory device using a variable resistor as a storage node and a stack-type diode as a switching unit will be described below with reference to FIGS. 1 and 2.
FIG. 1 is a circuit diagram illustrating a conventional non-volatile memory device using a diode as a switching unit. FIG. 2 is a plan view illustrating a structure of the non-volatile memory device of FIG. 1. Referring to FIG. 1, the conventional non-volatile memory device may include a circuit arrangement with a diode JD and a variable resistor R connected between a bit line BL and a word line WL. Referring to FIG. 2, the conventional circuit arrangement of FIG. 1 may be arranged on a semiconductor substrate (not shown). A variable resistor 55 may be disposed on an active region 52 of the semiconductor substrate, and a metal line 60 may be placed on the variable resistor 55 extending across the active region 52. The word line 50 may be connected to the active region 52, and the metal line 60 may act as a bit line BL.
A non-volatile memory device (e.g., a Phase-change Random Access memory (PRAM)), may occupy a cell area of at least 2F×2F, or 4F2, to process 2-bit data with a unit cell C as a reference. Accordingly, by employing a PRAM including a diode (e.g., such as the diode JD), the cell area for processing the unit-bit data may be decreased as compared with a non-volatile memory device that may use a transistor for the cell area to process the unit-bit data. However, a conventional PRAM using the diode may have difficulty performing multi-level operations.